Coding circuits are known from U.S. Pat. No. 3,984,863. In this circuit the frequency of the frequency-modulated signal occurs during a time interval in which the colour difference signal is zero and is made equal by the action of the first phase control loop to the so-called quiescent frequency, that is the frequency of the relevant reference signal generator. This is effected line-sequentially, that is to say, the signal of the oscillator assumes the relevant quiescent frequency, namely 4.40625 MHz or 4.250 MHz, respectively, at the beginning of each line period. To this end, a signal which has alternately these frequencies and which is compared with the frequency-modulated signal is applied to a phase detector. This results in a control voltage. This control voltage is applied to the frequency modulator circuit for controlling the instantaneous frequency of the frequency-modulated signal, causing this frequency to have the correct quiescent frequency, at least during the said time interval.
Thus, the known circuit accomplishes the stabilization of both quiescent frequencies, namely by means of two reference signal generators generating signals of these frequencies with a very high accuracy. The said patent specification proposes to provide a similar stabilization by means of a different construction by controlling a direct current component which is added to each colour difference signal. A square-wave signal is therefore added to the modulating signal, this signal having half the line frequency and assuming alternately the values corresponding with the relevant quiescent frequency. The amplitude of this signal is controlled, for which the two reference signal generators are still necessary.
In a still further construction only one accurate reference signal generator is required because the amplitude of the square-wave signal remains constant with very high accuracy.
It is an object of the invention to provide an encoding circuit of the above-defined type wherein one accurate reference signal generator is required for modulation, whereas the signal of an existing reference signal generator is used for the second control, which means a not inconsiderable saving. To this end, the encoding circuit according to the invention is characterized in that the frequency of the signal generated by the second reference signal generator is a harmonic of the line frequency and in that the second sample and hold circuit is operative for a period of time which is long relative to one period of the signal generated by the second reference signal generator.